Detection of radiation by the general process of exposing a thermoluminescent material to radiation then observing visible light emitted as the material is heated is well known. Exposing the thermoluminescent material to ultraviolet light prior to heating the thermoluminescent material is known to enhance the amount of visible light emitted. Medlin, U.S Pat. No. 3,388,252 uses this procedure to obtain a glow curve for quartziferous materials. Moran et al., U.S. Pat. No. 3,835,329 measures fast neutron flux by incorporating this procedure, and Miller et al., U.S. Pat. No. 4,954,707 uses it to "read" dosimeters using thermoluminescent material.
The thermoluminescent materials used in measuring radiation are crystalline compounds containing impurities or structural imperfections such as missing atoms or ions and mismatch between planes of the crystal lattices. Some of these imperfections are "traps" having the ability to capture or "trap" electrons and/or holes in excited states generated by exposure to ionizing radiation such as neutron, gamma and beta radiation. When trapped electrons are released, luminescence occurs. Release of electrons occurs upon heating the crystalline compound to a temperature that stimulates a trap to release electrons.
The purpose of the ultraviolet light exposure is to either load the traps with electrons or convert high temperature traps to low temperature traps in order to increase the amount of luminescence.
Conventional thermoluminescent dosimetry relies upon heating to high temperatures to stimulate traps to release electrons. At these high temperatures, radiation heat transfer or incandescence limits the sensitivity of the method. Other thermoluminescent dosimetry procedures including cryogenically cooled thermoluminescent dosimetry rely on exposing the thermoluminescent material to ultraviolet light to further stimulate the traps. Although the sensitivity is improved by using ultraviolet exposure compared to not using ultraviolet exposure, sensitivity is still limited to about 2.6 microcoulombs/Kg. Moreover, the addition of ultraviolet exposure adds to the complexity and expense of making the measurements. In all cases, the thermoluminescent material is exposed to radiation at about "room temperature" then heated to elevated temperatures or cooled to cryogenic temperatures and subsequently heated to obtain a reading.
Research identifying low temperature traps has been reported, for example, by VK Jain and MS Jahan Changes in the spectrum of Thermally Stimulated Emission From CaF.sub.2 :Mn at Low Temperatures, Phys. Stat. Sol. (b) 131, K161 (1985), and by R. Alcala et al. Manganese Centers in Low temperature X-Irradiated CaF.sub.2 :Mn, Phys. Stat. Sol. (b) 98, 315 (1980). Jain and Jahan irradiated calcium fluoride manganese with X-rays to a total dose of about 1.0 Mrad at a temperature of about and 83 K. to obtain a glow curve and emission spectrum. Alcala et al. irradiated calcium fluoride manganese with X-rays of 40 kV and 20 mA at a temperature of about 15 K. and 80 K. to observe differences in emission spectra.
Thermoluminescent dosimetry methods have not been widely used for environmental monitoring in the past because they have not had sufficient sensitivity to accurately measure environmental levels of radiation which may be as low as from about 5 microrad/hr to about 10 microrad/hr. In addition, thermoluminescent measurements prior to the present invention required laboratory analysis.
Environmental radiation detection is currently performed using large ionization chambers. The chambers are filled with a gas that becomes ionized upon exposure to ionizing radiation. Sensitivity to low exposures of ionizing radiation is obtained by providing a large active volume of gas. The ionization chambers provide real time information of the level of ionizing radiation. However, ionization chambers are too large and heavy to be used as a hand-held radiation detector.
It would be beneficial to exploit the small size and low weight advantages of cryogenic thermoluminescent radiation detection for environmental radiation detection without sacrificing sensitivity.